The present invention relates to processes for purification of molten salt electrolytes by passing a direct current between an anode and a cathode immersed in the electrolyte. Preferably, the electrolytes to be purified according to the invention are those containing magnesium chloride which are intended for use in the production of magnesium metal and chlorine gas.
Magnesium metal is usually obtained by electrolysis of molten magnesium chloride (MgCl2). The electrolysis reaction is carried out in one or more electrolysis cells, into which a molten salt electrolyte comprising magnesium chloride and one or more carrier salts is charged. In addition to magnesium metal, the electrolysis reaction also produces an off-gas of which the major component is chlorine (Cl2).
In order to optimize energy efficiency and recovery of magnesium metal from the electrolysis process, it is desired that the magnesium chloride fed to the electrolysis cells be substantially free of water and other oxygen-containing impurities. The presence of such materials in the molten salt electrolyte will result in diminished current efficiency, increased power consumption per tonne of magnesium produced, an increase in the rate of consumption of carbon electrodes used in the electrolysis cell, and reduced metal recovery due to increased sludge formation.
Water primarily enters the molten salt electrolyte during charging of magnesium chloride, which is not completely anhydrous. Any water which is not immediately removed from the molten salt electrolyte reacts with magnesium chloride to form magnesium oxide (MgO) and hydrogen chloride gas (HCl), which combine to form the soluble complex of magnesium hydroxychloride (MgOHCl). The presence of magnesium oxide and magnesium hydroxychloride in the molten salt electrolyte will have a detrimental effect on the subsequent electrolysis process and therefore it is desired that these compounds be removed as completely as possible. In presently used processes, water contained in the magnesium chloride feed is removed either by chlorine gas with addition of a reducing agent or by sparging hydrogen chloride acid gas into the molten electrolyte in order to ensure that the reaction equilibrium that results in the creation of magnesium oxide and magnesium hydroxychloride will be driven towards the destruction of these compounds.
Although the use of chlorine-based gases for the above purpose is somewhat effective, the complete destruction of magnesium oxide and magnesium hydroxychloride would require a large excess of the gas, and is therefore not practical. Therefore, even after this purification step, an amount of magnesium hydroxychloride typically remains in the molten salt electrolyte which will have a detrimental impact upon the operation of the electrolysis cell. Specifically, the presence of this compound will result in increased formation of sludge, the chief component of which is magnesium oxide, and generation of hydrogen gas in the electrolysis cell.
A number of processes for destruction of oxygen-containing impurities such as magnesium hydroxychloride are known in the prior art. Some of these are now discussed below.
U.S. Pat. Nos. 3,418,223 and 3,562,134 to Love relate to a continuous process for producing high purity magnesium and chlorine gas by electrolysis of anhydrous magnesium chloride salt. The process disclosed by Love starts by feeding solid blocks of substantially anhydrous magnesium chloride into a melt cell. A gas containing hydrogen chloride is fed into the melt cell through a pipeline to remove some of the surface moisture from the magnesium chloride. The melt cell contains two sets of electrodes: a first set of electrodes energized by an alternating current source and a second set of electrodes energized by a low voltage, direct current source of up to about 2 volts, the second set of electrodes being comprised of carbon. According to Love, the first set of electrodes melts the magnesium chloride blocks and the second set of electrodes decomposes oxygen-bearing compounds in the salt and converts the decomposed oxide to carbon monoxide gas which is evolved from the melt cell. Following purification, the magnesium chloride is transferred from the melt cell to a charging cell, from which it is transferred to the electrolysis cells.
U.S. Pat. No. 4,510,029 to Neelameggham et al. discloses a process for electrolytic purification of magnesium chloride in which direct current electrolysis is used to reduce impurities to low levels. Iron is mentioned as the primary impurity of interest in the Neelameggham patent. The purification process takes place in a steel tank having a refractory lining and provided with a main anode and a main cathode, between which are provided a number of bipolar electrodes having apertures provided therein for passage of electrolyte. Iron sludge is deposited on the cathodic faces of the bipolar electrodes, which are periodically removed for cleaning.
U.S. Pat. No. 2,375,009 to Lepsoe discloses a process for purification of molten magnesium chloride, which takes place in a purification furnace. Metallic magnesium carried by sludge removed in a settling furnace is transferred to the purification furnace. Lepsoe discloses that the metallic magnesium is effective in replacing metallic chloride impurities contained in the molten magnesium chloride.
U.S. Pat. No. 3,997,413 to Fougner is similar to Lepsoe in that the magnesium chloride is contacted with magnesium metal to remove impurities. However, in Fougner, the magnesium used for purification is in the form of a vapor which condenses on the molten salt. The condensed magnesium is intimately mixed with the magnesium chloride under conditions of vigorous agitation. According to Fougner, the purpose of the vaporized magnesium is to displace metal chloride impurities. Also mentioned is the use of hot molten cell bath material for admixture with the molten magnesium chloride in order to dry the magnesium chloride and reduce the amount of magnesium hydroxychloride produced by reaction of magnesium chloride with water.
U.S. Pat. No. 4,076,602 to Wheeler discloses a method for removing hydrogen and oxygen-containing impurities from magnesium chloride by feeding powdered, spray dried magnesium chloride to the electrolysis cell. Although the spray dried magnesium chloride contains water, magnesium hydroxychloride and magnesium oxide, Wheeler discloses that the powder melts instantaneously when it contacts the magnesium chloride and a substantial portion of the hydrogen-containing impurities are flash vaporized. The magnesium oxide and remaining water are kept in suspension by a high electrolyte circulation rate and are chlorinated by the cell chlorine produced at the anode.
In practice, it has been found that many methods for purification of magnesium chloride-containing electrolytes are either ineffective or impractical, and therefore the need remains for an effective method for purification of such molten salt electrolytes.
The present invention at least partially overcomes the disadvantages of the prior art by providing a process for purification of molten salt electrolytes in which oxygen-containing impurities are destroyed both electrolytically and chemically.
According to the process of the invention, a direct current is passed through a magnesium chloride-containing molten salt electrolyte. As in the Love process, the direct current results in destruction of hydroxychloride ions at the anode, thereby electrolyzing some of the magnesium hydroxychloride present in the electrolyte.
However, in contrast to the Love process, the direct current voltage and amperage in the process of the present invention are sufficient to cause electrolysis of a small proportion of the magnesium chloride present in the electrolyte to thereby produce finely dispersed magnesium droplets in the electrolyte. These droplets of magnesium metal react chemically with the magnesium hydroxychloride present in the electrolyte to produce magnesium chloride and an amount of magnesium oxide which is removed by settling from the electrolyte melt prior to the electrolysis reaction.